Advanced cathode material for lithium-ion battery is the key to promote the continuous development of the lithium-ion battery technology, even more the core technology of power battery replacement. Therefore, the development of low-cost, high-performance lithium iron phosphate (LiFePO4) has important practical significance. Although LiFePO4 cathode material has many outstanding advantages, there are still problems which are not completely solved, e.g. high cost of raw materials, low conductivity, poor magnification performance, low tap density. The following typical studies have been carried out to address these issues.
I. Researches on Improving the Preparation Process and Reducing the Cost
In recent years, it is reported that the precursor ferric phosphate (FePO4) was prepared first, and then reacted with lithium salt to prepare LiFePO4 cathode material, and some certain research results have been obtained. However, on the one hand, the cost for the iron source used in these preparation methods is higher, for example, using P2O5 and ferrous powder (Guan-nan Hao, Hao Zhang, Xiao-hong Chen, et al. A novel method for preparing pomegranate-structured FePO4/C composite materials as cathode for lithium-ion batteries [J]. Materials Research Bulletin, 47: 4048-4053, 2012), ferric chloride (Seunghoon Nam, Sungun Wi, Changwoo Nahm, et al. Challenges in synthesizing carbon-coated LiFePO4 nanoparticles from hydrous FePO4 and their electrochemical properties [J]. Materials Research Bulletin, 47: 3495-3498, 2012), ferric nitrate (Zhongqing Jiang, Zhongjie Jiang. Effects of carbon content on the electrochemical performance of LiFePO4/C coreshell nanocomposites fabricated using FePO4 polyaniline as an iron source [J]. Journal of Alloys and Compounds, 537: 308-317, 2012), ferrous chloride (Li Chen, Yongqiang Yuan, Xiafeng, et al. Enhanced electrochemical properties of LiFe1-xMnxPO4/C composites synthesized from FePO4.2H2O nanocrystallites [J]. Journal of Power Sourecs, 214: 344-350, 2012) and the like as the raw materials. Moreover, low-value by-product salts will be produced during the reaction process, such as NaNO3, NaCI and the like. Meanwhile, the by-product salts are present in the mother liquor produced from the reaction, as well as the washing liquid of iron phosphate, and will pollute the environment if being emitted directly. The processing thereof will increase the cost.
On the other hand, the prepared FePO4 precursor contains crystal water, which leads to the fact that the actual components cannot be reliably determined, and brings some difficulties to the precise batching. Moreover, since the water contained in the FePO4 precursor affects the oxygen content in the protective atmosphere, it is impossible to ensure that all of the ferric iron is reduced to ferrous iron during the sintering process. In addition, the products containing crystal water may produce moisture absorption or weathering phenomenon in the long-term storage process, so that the product components change over time, to adversely affect the stability of the process and product consistency.
Therefore, ferrous sulfate from the by-product of titanium dioxide as raw material, and inexpensive oxygen instead of commonly used hydrogen peroxide as an oxidant are used to prepare anhydrous FePO4 precursor by the introduction of microwave drying technology, which can achieve multiple purposes such as waste recycling, clean production, lowering the cost, and improving the electrochemical performance.
II. Research on Increasing the Conductivity
1. Doping and Coating Technology of Traditional Inactive Carbon Materials
At present, doping or coating various conductive agents is the main method to improve the conductivity of LiFePO4. Among them, researches on carbon coating are relatively more, and the technology thereof is relatively mature. When carbon coating of LiFePO4 is carried out, carbon can not only prevent oxidation of Fe2+ as a reducing agent, but also prevent the agglomeration of crystal grains and increase the conductivity of LiFePO4 as a conductive agent. The results show that LiFePO4/C composites in situ carbon coated have better electrochemical performance than those in situ carbon doped. The most commonly used carbon source materials include citric acid, sucrose, glucose, starch, organic acid and the like.
Jing Liu (Synthesis of the LiFePO4/C core-shell nanocomposite using a nano-FePO4/polythiophene as iron source[J]. Journal of Power Spurces, 197: 253-259, 2012) discloses using citric acid as carbon source to synthesize LiFePO4/C composites having a core-shell structure by the solid phase method and having an initial discharge specific capacity of 151 mAh/g at 0.1 C magnification. Shuping Wang (Shuping Wang, Hongxiao Yang, Lijun Feng, et al. A simple and inexpensive synthesis route for LiFePO4/C nanoparticles by co-precipitation [J]. Journal of Power Spurces, 233: 43-46, 2013) discloses using glucose as carbon source to synthesize LiFePO4/C composites having a carbon layer having a thickness of 2 nm by the coprecipitation method, and having a discharge specific capacity of 100 mAh/g at 10 C magnification, and having better magnification performance and cycle stability. Ching-Yu Chiang (Ching-Yu Chiang, Hui-Chia Su, Pin-Jiun Wu, et al. Vanadium substitution of LiFePO4 cathode materials to enhance the capacity of LiFePO4-based lithium-ion batteries [J]. The Journal of Physical Chemistry. 116: 24424-24429, 2012) discloses using LiOH, FeC2O4.2H2O, NH4.H2PO4 and citric acid as raw materials, saccharose as the carbon source, sintering at a high temperature of 800° C. to prepare vitriol-doped LiFePO4/C composite cathode material by the modified sol-gel process, having a discharge specific capacity of 155 mAh/g at 0.1 C Magnification.
2. Doping and Coating Technology of Active Carbon Materials
Recent studies have shown that doping or coating active carbon materials, such as conductive polymers, graphene and carbon nanotubes, can effectively enhance the conductivity and electrochemical performance of LiFePO4.
Polypyrrole having a theoretical specific capacity of 72 mAh/g can act as a conductive agent on the surface of LiFePO4 and enhance its conductivity, and also exhibit the activity of the cathode material. Therefore, it is highly concerned by researchers at home and abroad. Research by Hai Jia (Preparation and Performance research of Polypyrrole-coated LiFePO4 [J]. Power Technology, 36(11): 1610-1613, 2012) shows that LiFePO4 prepared by the solvothermal method has a discharge specific capacity of 105.6 mAh/g at 1 C magnification after coating with PPy. Yi-Ping Liang (Hydrothermal synthesis of lithium iron phosphate using pyrrole as an efficient reducing agent [J]. Electrochimica Acta, 87(1): 763-769, 2013) discloses LiFePO4/PPy composites prepared by the hydrothermal method has a discharge specific capacity of 153 mAh/g at 0.2 C magnification. The addition of PPy not only increases the conductivity of LiFePO4 as a conductive agent, so as to improve the cycle performance, but also acts as a reducing agent during the subsequent sintering process, so as to reduce Fe3+ produced during the reaction into Fe2+.
Since the discovery by K S Novoselov in 2004, the application researches on graphene in various fields have attracted significant attentions due to its remarkable advantages, such as high specific surface area, high conductivity, high mechanical strength and good softness. In recent years, the doping modification researches of graphene on LiFePO4 cathode material have also made good progress. Xiaohua Ma (Study on the Electrical Properties of Nanocrystalline Lithium Iron Phosphate/Graphene/Carbon Composites [J]. New Materials Industry, 2:71-75, 2011) from Fudan University discloses using ethylene glycol as a solvent to in situ synthesize a nanoscale LiFePO4/Graphene/Carbon Composite with good uniformity at 250° C. by the solvothermal method, so that LiFePO4 is uniformly dispersed on graphene sheet; graphene provides passageway to Li+ migration and enhances the electrochemical performance of the composites.
Carbon nanotubes have high conductivity, large specific surface area utilization efficiency, and have lithium storage performance. The application thereof in LiFePO4 has become a hotspot. Research from Youyi Peng (A Study on the LiFePO4/MWCNTs cathode materials for Li ion batteries [J]. Electrochemistry, 15(3):331-335, 2009) shows that LiFePO4/MWCNTs electrode having a MWCNT content of 10 wt. % has better charge-discharge performance than other composite electrode having other ratios, and has small polarization, strong stability, more stable charge-discharge platform, and higher conductivity.
In view of the high cost, poor conductivity and cycle stability of LiFePO4 cathode materials, there are still some problems in the improvement of the latest researches at home and abroad. In the raw materials used for the preparation of LiFePO4, most of the iron sources are oxalate, nitrate, organic salts and the like. Phosphates are obtained more from ammonium salts thereof, which will lead to increased product cost, and even generation of environmentally unfriendly gas, such as nitrogen oxides and ammonia, so as not to be conducive to large-scale industrial applications. Iron source in the raw materials used in the synthesis route of novel FePO4 precursor has a high cost; the FePO4 precursor contains crystal water, which brings some difficulties to the precise batching and affects the process stability. Although coating carbon on the surface of LiFePO4 can effectively increase the electronic conductivity, limit further increase of the particle size and achieve the object of controlling the particle size, the addition of carbon will decrease the tap density, so as to further reduce the volume and mass specific energy since carbon has a lighter mass and is non-active substance.
Recently, it has been reported that polypyrrole and graphene are compounded to prepare supercapacitor electrode materials; graphene and carbon nanotubes are compounded, and polypyrrole, graphene and carbon nanotubes are compounded to prepare gas-sensitive materials. However, there is no report as of now that polypyrrole, graphene, carbon nanotubes and conventional non-active carbon materials have been commonly doped to increase the conductivity and electrochemical performance of LiFePO4 by the synergistic effect therebetween.